Current source circuits with common mode noise rejection

ABSTRACT

An improved current source circuit is adapted for use with two-wire transmission systems of the type encountered in telephony, and has excellent common mode noise rejection. In its various embodiments, the current source provides an interface for coupling signals with a telephone pair, operates to cancel or negate the effect of unwanted impedances on the pair, generates a selected impedance across the pair, and functions as an electronic hybrid circuit for interfacing between a two and a four-wire telephone transmission facility, all while affording common mode rejection to noise on the line. In addition, the various embodiments of the current source are combined into an improved terminating set for a two to four-wire telephone transmission facility.

BACKGROUND OF THE INVENTION

The present invention relates to current sources in general, and inparticular to an improved current source for use with two-wiretransmission systems and which has excellent common mode rejection tonoise.

Metallic noise across two-wire telephone transmission pairs isundesirable since it affects both the quality of transmission and properoperation of equipment connected with the line. As is well known, todecrease the effect of metallic noise it is desirable to have bothlongitudinal balance and low impedance to ground in the tip and ringcircuits of a telephone pair.

Nulling out equal voltages on the line of a telephone pair is calledcommon mode noise rejection. It is well known, for example, that a wellbalanced, twisted telephone pair operating metallically providesrejection to noise inductively coupled from power lines. This isaccomplished not only by the twisting imparted to the pair, but also bytheir having balanced longitudinal impedances to ground and equal seriesimpedances.

Although much can be done to the tip and ring leads to enhance commonmode rejection to noise, with prior art techniques some longitudinalnoise nevertheless remains. In addition, equipment interfacing with thetelephone pair can cause longitudinal unbalance and thereby increasenoise on the pair. It is therefore very desirable to provide means forinterfacing with a telephone pair without degrading the longitudinalbalance thereon, while actually enhancing common mode rejection to noiseby means of a low impedance balanced path to ground.

Since undesirable impedances often exist in a telephone pair, and sinceit is usually necessary to have a selected impedance across the pair, itwould also be advantageous if the means for interfacing cancelledunwanted impedances from the pair while exhibiting a desired impedanceacross the pair.

Hybrid circuits are used in long distance telephone transmissionfacilities to interface between two and four-wire circuits. Thefour-wire circuit comprises one pair of wires called a receive channeland another pair called a transmit channel, and the conventionaltwo-coil hybrid circuit couples signals from a distant subscriber on thereceive channel with the two-wire circuit leading to a local subscriber,and couples signals from the local subscriber on the two-wire circuitwith the transit channel leading to the distant subscriber. It wouldtherefore also be advantageous to provide an improved and accurateelectronic hybrid circuit which provides common mode rejection tometallic noise on telephone pairs.

It is well known that current sources provide exceptional accuracy andwould be very desirable circuit mediums or interface units for telephoneapplications, except that conventional current sources exhibit no commonmode rejection to noise, but instead introduce longitudinal unbalanceonto a telephone pair. Such undesirable characteristics of conventionalcurrent sources severely restrict their use in telephony, despite thefact that current sources otherwise exhibit other properties which wouldenhance their use.

OBJECTS OF THE INVENTION

An object of the present invention is to provide an improved currentsource for interfacing with a two-wire transmission circuit whileproviding common mode rejection to noise on the circuit.

Another object of the present invention is to provide such a currentsource which exhibits a selected input impedance to the two-wiretransmission circuit.

A further object of the invention is to provide such a current sourcewhich cancels an unwanted impedance from the two-wire transmissioncircuit.

Yet another object of the invention is to provide such a current sourcewhich operates as an electronic hybrid circuit for interfacing betweentwo and four-wire telephone transmission facilities.

A still further object of the invention is to provide such a currentsource for use in a terminating set between two and four-wire telephonetransmission facilities.

SUMMARY OF THE INVENTION

In accordance with the present invention, a current source is providedfor interfacing with a two-wire transmission system, for example withthe tip and ring leads of a telephone pair, for generating a controlledcurrent flow through the system and for providing common mode rejectionto longitudinal voltages on the system. Said current source has firstand second outputs each for connection with a separate one of the wires,and comprises first and second voltage amplifier means each having aninput, an output and an output impedance connected between the amplifiermeans output and a separate one of said current source outputs. Meansare included for connecting said first amplifier means output with saidsecond amplifier means input so that voltage signals at said firstamplifier means input control the voltages generated at each of saidamplifier means outputs, and feedback means is coupled between saidcurrent source first output and said first amplifier means input. Inaddition, circuit means is coupled between said current source outputsand said first amplifier means input, and the arrangement is such thatsaid feedback means and said circuit means couple signals at saidcurrent source outputs, and therefore on the wires of the transmissionsystem, to said first amplifier means input to operate said first andsecond amplifier means so that the same generate equal voltages at theiroutputs in response to equal longitudinal voltages on the wires, wherebysaid current source has common mode rejection to equal longitudinalvoltages.

In its various embodiments the current source provides an interface forcoupling signals with a two-wire transmission system, cancels or negatesthe effect of unwanted impedances in the system, generates a selectedimpedance in the system, and functions as an electronic hybrid circuitfor interfacing between the two-wire system and a four-wire transmissionfacility, all while affording common mode rejection to noise on theline. In addition, the various embodiments of the current source arecombined into a terminating set for interfacing between a two and afour-wire transmission facility.

The foregoing and other objects, advantages and features of theinvention will become apparent upon a consideration of the followingdetailed description, when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a preferred embodiment ofcurrent source in accordance with the teachings of the presentinvention, showing the same connected with a network representation ofthe tip and ring leads of a two-wire telephone circuit;

FIG. 2 is a schematic circuit representation of an embodiment of thecurrent source wherein the same effectively cancels an impedance on thetip and ring leads of the telephone circuit.

FIG. 3 is a schematic circuit representation of an embodiment of thecurrent source wherein it provides a selected input impedance to the tipand ring leads of the telephone circuit;

FIG. 4 illustrates partly in schematic and partly in block diagram forman arrangement of the current source which forms an electronic hybridcircuit for interfacing between a two and four-wire telephonetransmission facility, and

FIG. 5 illustrates partly in schematic and partly in block diagram forma terminating set formed with the current source of the invention, forinterfacing between a two and a four-wire telephone transmissionfacility.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown within dashed lines and indicatedgenerally at 10 a novel current source circuit in accordance with apreferred embodiment of the invention. For the purpose of illustratingoperation and use of the current source, the same is shown connected orinterfacing at outputs 12 and 14 therefrom with a pair of telephonewires comprising a tip lead T and a ring lead R. To schematicallyrepresent metallic signal and impedance conditions that normally existacross such tip and ring leads, a metallic voltage source V_(m) and aload R_(m) are shown connected in series between the leads, and torepresent longitudinal noise which is inherent on the leads alongitudinal voltage generator V_(L) in series with a longitudinalcapacitance C_(L) is connected between ground and the tip lead through atip lead to ground impedance Z_(tg) and between ground and the ring leadthrough a ring lead to ground impedance Z_(rg). To facilitate adescription of the current source, switch contacts S are illustrated forconnecting and disconnecting the longitudinal voltage and impedanceswith and from the tip and ring leads, although it is understood that inpractice such longitudinal voltage and impedances cannot bedisconnected. For the purpose of decreasing longitudinal noise the tipand ring leads are usually twisted about each other to form a twistedtelephone pair, and it is assumed that the impedances Z_(tg) and Z_(rg)are equal so that the leads have balanced longitudinal impedances toground.

The current source of the invention interfaces with the tip and ringleads, connects signals therewith and operates in a manner that providescommon mode rejection to noise on the leads. That is, the current sourcenulls out equal common mode noise or longitudinal voltages on the tipand ring leads, but does not affect the differential or metallic voltagethereacross. To this end, the current source includes an operationalamplifier (op amp) A1 connected at its output with the tip lead Tthrough a resistor R1 in series with a capacitor C1, and an op amp A2for receiving at its inverting input a voltage from a signal source V1through a resistor R2. The signal source VI may represent, for example,speech signals from a subscriber'handset, which are to be applied acrossthe tip and ring leads. A non-inverting input to the op amp A2 isconnected to reference or ground potential and the output from the opamp is applied both to its inverting input through a resistor R3 and toan inverting input to the op amp A1 through a resistor R4. Anoninverting input to the op amp A1 is coupled to reference potentialand a potentiometer R5 is between the output from and the invertinginput to the op amp. A resistor R6 provides feedback from the juncturebetween the capacitor C1 and the resistor R1 to the inverting input tothe op amp A2, and the inverting input to the op amp A1 receives througha resistor R7 a voltage signal from a signal source V2. Similar to thesignal source V1, the signal source V2 represents a signal to be appliedacross the tip and ring leads, except as compared with a signal from thesource V1, and as will be described, the signal across the tip and ringleads from the source V2 will be inverted.

To the extent described the circuit comprising the resistors R1-R7 andthe op amps A1 and A2 comprises a conventional current source of a typeknown in the art. Because of the double inverting function performed bythe op amps A1 and A2 the signal source V1 generates a noninvertedoutput current I₀, and the signal source V2 an inverted current at theoutput from the op amp A1. It is understood, of course, that thisconventional current source is a single ended current source notsuitable for direct coupling to telephone lines, but for the purpose offacilitating an understanding of the current source of the invention, aswill hereinafter be described, the operation of the conventional currentsource will first be considered.

In use of the circuit thus far described, and in the overall invention,the normal scaling of the various resistors is such that the resistorsR3, R4, R5 and R6 have equal values, and the resistor R6 provides 100%positive feedback for any output voltage V_(A1) from the op amp A1 minusthe voltage drop across the resistor R1. Therefore, the op amp A1 willgenerate at its output a voltage which causes a prescribed outputcurrent I₀ to flow through the resistor R1 in order to reduce thepositive feedback through the resistor R6. For example, if the voltageV1 at a noninverting input 16 to the current source equals 1.0 volt, thevoltage V2 at an inverting input 18 to the current source equals 0volts, and the values of the resistors R2 and R3 are equal, then thevoltage V_(A1) will rise sufficiently high to generate an output currentflow which causes a 1.0 volt drop across the resistor R1. Thus, if theresistor R1 equals 1000 ohms, then 1.0 milliamp of output current I₀will be produced for each volt generated by the signal source V1.Mathematically, the current at the output from the op amp A1 may beexpressed as follows:

    I.sub.0 =V1R5/R2R1-V2R5/R7R1.

Because of the resistor R6, it is apparent that only a part of theoutput current I₀ is delivered to a load, for example the load R_(m),since the output current is split between the load and the resistor R6.

To provide a double ended current source for interfacing with the tipand ring leads T and R, in accordance with the teachings of the presentinvention the circuit also includes an op amp A3 connected at aninverting input thereto with the output from the op amp A1 through aresistor R8 and having a resistor R9 connected between its output andinverting input. A noninverting input to the op amp A3 is connected toreference potential, and its output is coupled with the ring lead Rthrough a resistor R10 in series with a coupling capacitor C2 and withreference potential through a resistor R13. If in the double endedcircuit the resistor R8 equals the resistor R9, the resistor R1 equalsthe resistor R10 and the coupling capacitors C1 and C2 are neglected,then the current I_(rm) delivered to the load resistance R_(m) is

    I.sub.rm =I.sub.0 R6R.sub.m /R.sub.m (R6+R.sub.m)

Also, an input impedance Z_(in) to the current source would then beessentially equal to the value of the resistor R6, and the outputvoltages from the op amps A1 and A3 would not permit any currentgenerated by the metallic source V_(m) to flow through either of theresistors R1 and R10.

In order to provide the current source with common mode rejection tolongitudinal noise in the tip and ring leads, the invention alsocontemplates connection of a pair of equal value resistors R11 and R12in series between the juncture of the resistor R1 and the couplingcapacitor C1 and the juncture of the resistor R10 and the couplingcapacitor C2, with a juncture between the resistors R11 and R12 beingcoupled with the inverting input to the op amp A1. In this manner, avoltage signal generated by the signal source V1 will still generate anoutput current I₀ as shown, and a voltage signal generated by the signalsource V2 an inverted output current, but the current source willnevertheless have common mode rejection to noise.

To this point it has been assumed that the switch S was open so thatthere were no longitudinal voltages or impedances on the tip and ringleads. Obviously, such noises and impedances exist on actual telephonecircuits, and for the purpose of describing the common mode rejection tonoise of the current source it is now assumed that the switch S isclosed so that each of the tip and ring leads has a longitudinal voltageas well as a longitudinal impedance to ground. In practice the inherentlongitudinal impedances Z_(tg) and Z_(rg) are substantially equal, andnoise suppression results when the longitudinal voltages induced on thetip and the ring leads are both as low as possible and as equal aspossible. Noise or common mode voltage is an unwanted tip to ringpotential, and the first requirement for noise suppression is balancedtip lead to ground and ring lead to ground impedances.

For balanced tip lead to ground and ring lead to ground impedances, thevalues of the capacitors and resistors are selected so that:

    C1=C2, R1=R10, R6=R13, R8=R9, and

    R11=R12=2R6=2R13,

which not only provides the necessary longitudinal impedance balance,but also disables the outputs of the op amps A1 and A3 so that the sameare 0 volts or ground in response to equal longitudinal voltages on thetip and ring leads. When the outputs from the op amps A1 and A3 are 0volts to longitudinal voltages, then both the tip and ring leadlongitudinal voltages are decreased since the current source provides arelatively low, balanced impedance path from the tip lead to ground andfrom the ring lead to ground.

When equal common mode voltages are applied to the tip and ring leads,the foregoing scaling of the resistors and capacitors results in acurrent flow through the resistors R11, R12 and R4 the sum of whichequals zero, or:

    I.sub.R11 +I.sub.R12 +I.sub.R4 =0

Consequently, with the established relationship the sum of the currentflow at the inverting input to the op amp A1 is zero and the outputvoltages from the op amps V_(A1) and V_(A3) are each equal to zero tolongitudinal voltages, whereby common mode rejection to noise isobtained from the current source 10.

The foregoing result is perhaps best explained by way of example.Assume, for instance, that the longitudinal voltages of the tip and ringleads are such that one volt is applied to the node defined by thecapacitor C1 and the resistors R1, R6 and R11, and one volt is alsoapplied to the node defined by the capacitor C2 and the resistors R10,R12 and R13. Under this condition, the following current flows occur:

    I.sub.R11 =1/R11,

    I.sub.R12 =1/R12, and

    I.sub.R4 =-R3/R6R4.

Since from the previously defined resistor values

    R3/R6=1, and

    R11=R12=2R4,

the current at the inverting input to the op amp A1 sums to zero andtherefore the voltages at the outputs from the op amps A1 and A3 equalzero. Thus, common mode rejection to longitudinal noise is offered bythe current source 10.

Since the outputs from the op amps A1 and A3 are zero to longitudinalvoltages on the tip and ring leads, but not to metallic voltages, thecurrent source decreases the tip and ring lead longitudinal voltages andgenerates a low value, balanced impedance path from the tip lead toground and from the ring lead to ground. To this end, the longitudinalimpedance from the tip lead to ground is ##EQU1## and from the ring leadto ground is ##EQU2## Obviously, the total impedance presented to thelongitudinal voltage by the current source is equal to the parallelcombination of the tip to ground and the ring to ground longitudinalimpedances, or

    Z.sub.1 =Z.sub.tg Z.sub.rg /(Z.sub.tg +Z.sub.rg)

Consequently, for equal common mode voltages on the tip and ring leads Tand R the sum of the currents at the inverting input to the op amp A1 iszero and the output voltages from the op amps A1 and A3 are zero,whereby the current source provides common mode rejection to thelongitudinal voltages on the tip and ring leads.

Another important attribute of the current source of the invention isits accuracy in providing a signal from the source V1 and/or V2 onto thetip and ring leads. If the values of the resistors are set so that theresistors R3, R4, R6 and R13 all have equal values, and so that thevalue of the resistor R8 equals that of the resistor R9, then the erroror leakage current I_(leak) flowing through the resistor R6 equals

    I.sub.leak =1/R1+1/R6-R5/R1R6.

Accordingly, the leakage or error current I_(leak), or that portion ofthe output current I₀ flowing through the resistor R6, can at least intheory be eliminated by a proper selection of the value of the resistorR5.

The leakage current I_(leak) through the resistor R6 can become quitesignificant if the impedances of the coupling capacitors C1 and C2 areconsidered at low frequencies. A normal method of reducing the leakagecurrent would be to make the values of the resistors R6 and R3 verylarge. However, this strategy backfires because the offset voltage of opamps is much greater when large value resistors are used, and inpractice the value of the resistor R5 cannot be set so that the leakagecurrent is equal to zero. Nevertheless, through the proper selection ofop amp resistors, etc., the value of the leakage current can be made tovery closely approach zero, and in the disclosure which follows it isassumed that the resistor R5 has been adjusted to reduce the leakagecurrent I_(leak) to a sufficiently low value so that it can, for allpractical purposes, be ignored. To this end, the adjustment of theresistor R5 which, at least in theory, reduces the leakage current tozero, occurs when the value of the resistor R5 equals the sum of thevalues of the resistors R1 and R6.

It is worth noting that the input impedance Z_(in) of the current sourceto the metallic voltage can theoretically be made infinite, so that thecurrent source does not in any way load down the metallic signal on thetip and ring leads. In particular, the metallic input impedance Z_(in)of the current source is equal to

    Z.sub.in =1/(1/R1+1/R6-R5/R1R6).

Accordingly, the basic current source of the invention provides anefficient and improved means for interfacing with the tip and ring leadsof a two-wire telephone circuit while providing common mode rejection tolongitudinal noise on the circuit, for generating a selected and ifdesired substantially infinite input impedance to the metallic voltageon the circuit, while simultaneously permitting connection of externalsignals with the circuit as represented by the voltages from the signalsources V1 and V2. In this connection, the output current I₀ provided bythe current source in response to voltage signals from the signalsources V1 and V2 is

    I.sub.0 =V1R5/R1R2-V2R5/R1R7,

and the current I_(rm) through the metallic load R_(m) in response tothe output current I₀ is

    I.sub.rm =V1R6/R1R2-V2R6/R1R7,

when the component values of the current source have been adjusted sothat the leakage current through the resistor R6 is insignificant.

The current source of FIG. 1 thus provides common mode rejection tonoise on the tip and ring leads of a two-wire telephone transmissionsystem while allowing a current signal to be introduced onto the lineswhich is a very accurate representation of voltage signals generated bythe signal sources V1 and/or V2. At the same time, the current sourcepresents a selected input impedance to the circuit. The current source,however, has significant additional versatilities in use with telephonecircuits, and is capable of a number of modes of operation, all of whichenhance its use in telephony.

For example, in the embodiment of the current source as shown in FIG. 2,the same may be interfaced with the tip and ring leads of the telephonecircuit and operated in such a manner as to effectively eliminate orcancel an unwanted impedance across the tip and ring leads. One suchtype of impedance is a battery fed relay coil or, as shown in FIG. 2, aninductively isolated battery feed comprising a series connection of apair of resistors R14 and R15 with an inductor L1 in line with a batteryB across the tip and ring leads. Conceptually, the impedance presentedby the inductor L1 and the resistors R14 and R15 can be effectivelyeliminated or cancelled if the current source supplies all of thecurrent that would otherwise be required by the impedance should ametallic voltage be applied across the tip and ring leads, so that themetallic voltage or signal source does not supply such current and theimpedance provided by the inductor L1 and resistors R14 and R15 appearsas an open circuit to the metallic voltage. Of particular interest isthe fact that the impedance can be eliminated or cancelled metallically,yet remain undisturbed longitudinally.

Referring in particular to FIG. 2 wherein like reference numerals denotelike components, impedance cancellation is accomplished by replacing theresistor R2 and noninverting signal source V1 of FIG. 1 with animpedance cancelling circuit, indicated generally at 20, which includesa resistor R16 and on op amp A4. An inverting input to the op ampconnects with the juncture between the resistor R14 and the inductor L1through a resistor R17 and a capacitor C3, with the output from the opamp through a resistor R18, and with the ring lead R through a resistorR19 and a capacitor C4. A noninverting input to the op amp connects withground or reference potential through a resistor R20, with the tip leadT through a resistor R21 and a capacitor C5, and with the juncturebetween the resistor R15 and the coil L1 through a resistor R22 and acapacitor C6.

To eliminate the metallic impedance of the inductor L1 and the resistorsR14 and R15, the component values of the impedance cancelling circuit 20are chosen so that equal voltages on the tip and ring leads do notproduce a voltage V_(A4) at the output from the op amp A4. To have 0volts at the output from the op amp A4 in response to equal tip and ringlead voltages, and also to have longitudinal balance and common moderejection to noise, the component values are selected so that thecapacitors C3, C4, C5 and C6 are equal, the resistors R17, R19, R21 andR22 are equal, the resistors R14 and R15 are equal, and the resistorsR18 equals the resistor R20. In addition, the values of the resistorsR14, R15, R17 and R21 are quite large, so that in operation of thecircuit the loading effect of those resistors, and therefore of thecapacitors C3-C6, is negligible. In addition, the value of the resistorsR17, R19, R21 and R22 is chosen to be much greater than the capacitivereactance of the capacitors C3-C6 at the frequency of operation of thecircuit.

The output voltage V_(A4) from the op amp A4 is proportional to acurrent I flowing through the unwanted impedance comprising the inductorL1 and the resistors R14 and R15. To demonstrate this relationship, thevoltage on the tip lead is defined as V_(t), that at the juncture of theresistor R14 and the inductor L1 as V_(t) ', that on the ring lead asV_(r) and that at the juncture between the resistor R15 and the inductorL1 as V_(r) ', with these designations, the voltage V_(A4) at the outputfrom the op amp A4 may be expressed as:

    V.sub.A4 =(V.sub.t -V.sub.t '+V.sub.r '-V.sub.R)R18/R17,

and the current I flowing through the impedance comprising the inductorL1 and the resistors R14 and R15 is:

    I=(V.sub.t -V.sub.t ')/R14=(V.sub.r '-V.sub.r)/R15.

Since the op amp A4 has replaced the signal source V1 of FIG. 1 then,from the equations developed with respect to FIG. 1, for the circuit ofFIG. 2 the output current I₀ flowing through the resistor R1 of thecurrent source 10 is:

    I.sub.0 =V.sub.A4 R5/R1R16,

and the current I_(rm) delivered to the load, which is the outputcurrent I₀ less any leakage current I_(leak) through the resistor R6,is:

    I.sub.rm =V.sub.A4 R6/R1R16

When the load current I_(rm) delivered by the current source to theunwanted load impedance comprising the resistors R14 and R15 and theinductor L1 is made equal to the current I that would otherwise flowthrough the load in response to a metallic voltage across the tip andring leads, the metallic voltage source does not generate any currentflow through the impedance and the impedance appears as an open circuitto the metallic votlage, whereby the impedance is effectively cancelledwith respect to the metallic voltage source. Cancellaton occurs when thevalue of the resistors are adjusted so that:

    I.sub.rm =I, and

    R5=R1+R6.

It is worth noting that the value of the inductor does not enter intoimpedance cancellation. Accordingly, the inductor may have any value,its value may change with a d.c. current flow therethrough, age, etc.,and yet the circuit nevertheless automatically eliminates the metallicimpedance presented by the inductor L1 and the resistors R14 and R15.Obviously, the circuit may be used to eliminate any unwanted metallictip to ring lead impedance without disturbing their longitudinalimpedances.

FIG. 1 illustrates use of the basic current source 10 as an interfacefor introducing signal voltages, for example speech signals, onto thetip and ring leads of a two-wire telephone circuit, while at the sametime providing common mode rejection to noise. In FIG. 2 the currentsource similarly has common mode noise rejection, and includes animpedance cancelling circuit 20 for effectively eliminating an unwantedmetallic impedance from between the tip and ring leads. As described inrespect of the embodiment in FIG. 1, which description also applies tothe embodiment of FIG. 2, the input impedance Z_(in) looking into thecurrent source may, with the resistor R5 properly adjusted, betheoretically made infinite. The embodiment shown in FIG. 3, however,demonstrates use of the invention to electronically and accuratelygenerate any selected input impedance Z_(in), which conventionally isordinarily accomplished with a transformer coupled voltage source acrossthe tip and ring leads. Unfortunately, in such conventional practice thesize, cost, leakage reactance, magnetizing current, etc., of atransformer creates considerably greater error than is desired. Suchdeficiences are not present in the circuit of FIG. 3.

Referring to FIG. 3, the circuit thereof includes the basic currentsource of FIG. 1 in which like reference numerals have been used todenote like components, and the operation of which is the same asdescribed in respect of FIG. 1. The circuit of FIG. 3 further includes,however, a tip to ring voltage detecting circuit, indicated generally at30, comprising a differential op amp A5 connected at its noninvertinginput with the tip lead through a resistor R23 and a capacitor C7 and atits inverting input with the ring lead through a resistor R24 and acapacitor C8. A resistor R25 is between the noninverting input to the opamp and ground, and a resistor R26 is between the inverting input to andthe output from the op amp. The particular connections cause the op ampto operate as a differential amplifier and to exhibit common moderejection to noise.

In selecting the parameters of the circuit, the values of the resistorsR23 and R24 are chosen to be sufficiently high so as to present anegligible load, and the values of the resistors R23 and R24 and of thecapacitors C7 and C8 are chosen so that the resistors present aconsiderably higher impedance than do the capacitors at the frequency ofoperation of the circuit. In addition, the capacitor C7 is equal invalue to the capacitor C8, the resistors R25 and R26 are equal, and thevalues of the resistors R23 and R24 are equal and significantly greaterthan either the load impedance R_(m) or a desired input impedance Z_(in)to the circuit.

The output voltage V_(A5) from the op amp A5 is dependent upon its gain.To this end, the gain G_(A5) of the op amp A5 equals R26/R24 or R25/R23.Accordingly, the output voltage V_(A5) is equal to the voltage V_(tr)between the tip and ring leads times the gain of the op amp, or:

    V.sub.A5 =V.sub.tr G.sub.A5

In order to generate a selected input impedance Z_(in), the output fromthe op amp A5 is connected either with the inverting input to the op ampA1 through an impedance Z27 or with the inverting input to the op amp A2through an impedance Z27'. In the case where the output from the op ampA5 is connected with the op amp A1, the op amp A5 and impedance Z27essentially take the place of the signal source V2 and resistor R7 ofFIG. 1, and cause an inverted output current I₀ to be generated by theop amp A1. The inverted output current represents a current that wouldbe drawn by a positive impedance, whereby through proper selection ofcomponent values for the circuit a positive input impedance Z_(in) tothe circuit, or a positive impedance across the tip and ring leads maybe generated. In the case where the output from the op amp A5 is appliedto the inverting input to the op amp A2, then the op amp A5 andimpedance Z27' essentially take the place of the signal source V1 andresistor R2 of FIG. 1 and cause a noninverted output current I₀ to begenerated by the op amp A1. The noninverted output current represents acurrent flow that would be provided by a negative impedance, wherebythrough proper selection of component values for the circuit a negativeinput impedance Z_(in) to the circuit, or a negative impedance acrossthe tip and ring leads, may be generated.

In accomplishing the foregoing, and considering the relationship ofcomponent values previously set forth, to generated a positive inputimpedance Z_(in) the output from the op amp A5 is connected with theinverting input to the op amp A1 through the impedance Z27 and the valueof the impedance Z27 is selected such that

    Z27=R6G.sub.A5 Z.sub.in /R1,

where Z_(in) is the value of the desired positive impedance. Should anegative input impedance be desired, then the output from the op amp A5is applied to the inverting input to the op amp A2 through the impedanceZ27' and the value of the impedance Z27' is chosen such that

    Z27'=R6G.sub.A5 Z.sub.in /R1,

where Z_(in) represents the value of the desired negative impedance.

Consequently, simply by selecting appropriate values for the impedancesZ27 or Z27', positive or negative impedances of any selected value maybe generated at the input to the current source and across the tip andring leads. It is understood, of course, that the various embodiments ofthe invention thus far described may be combined so that, if desired, asshown in FIG. 2 an undesired impedance can be eliminated from across thetip and ring leads while, as shown in FIG. 3, a selected terminatingimpedance can simultaneously be generated across the leads, all thewhile offering common mode rejection to longitudinal noise on the leads.

FIG. 4 illustrates another embodiment of the invention, wherein thecurrent source is modified to operate as an electronic hybrid circuit.As is known in the art, conventional hybrid circuits of the two-coilvariety are used to interface between four-wire long distance telephonetransmission circuits and two-wire circuits leading to localsubscribers. In its operation, the hybrid circuit transfers voicesignals on a receive channel of the four-wire circuit to the two-wirecircuit without, at least theoretically, coupling signals on the receivechannel with a transmit channel of the four-wire circuit, and connectssignals on the two-wire circuit with the transmit channel. Any crosscoupling of signals from the receive to the transmit channel isundesirable, since such coupled signals comprise an echo signal which isreturned, unless otherwise eliminated, to the distant subscriber.Although conventional two-coil hybric circuits are reasonably accuratefor trans-hybrid loss, the transformers for the circuits are expensive,bulky and do not provide as satisfactory a tip to ring lead inputimpedance as does the present invention when used as an electronichybrid circuit.

For the purpose of clarity, in FIG. 4 the basic current source 10 shownin FIG. 1 and the voltage detecting circuit 20 of FIG. 3 are illustratedin block diagram form. Actually, the current source and the voltagedetecting circuit are arranged substantially as shown in FIG. 3, exceptthat the input 18 to the current source is not used and the output fromthe voltage detecting circuit is not applied as an input to the currentsource.

More particularly, as shown in FIG. 4 the tip and ring leads T and R ofa two-wire transmission circuit leading to a local subscriber have aload impedance Z_(m) and a voltage source V_(m), representative of theimpedance and voltage signals generated by the local subscriber's handset, connected in series thereacross. The inputs to the voltagedetecting circuit 30 and the outputs from the current source 10 areconnected across the tip and ring leads, and the current source receivesat its noninverting input 16 voltage signals from the signal source V1,which represent speech signals from a far end subscriber on the receivechannel.

An op amp A6 has its output connected with the transmit channel leadingto the far end subscriber, a noninverting input to the op amp isreferenced to ground, its inverting input is coupled with the outputfrom the voltage detecting circuit 30 through a resistor R28, and aresistor R29 is between the inverting input to and the output from theop amp. Intermediate the receive and transmit channels is an electronichybrid circuit, indicated generally at 40, comprising an op amp A7coupled at its inverting input with the signal on the receive channelthrough a resistor R30 and at its output with its noninverting inputthrough a resistor R31. A resistor R32 is between the inverting input toand the output from the op amp A7, and the noninverting input to the opamp is coupled with reference potential through a resistor R33. Animpedance Z_(m) ' is connected between ground and the output from the opamp A7 through the resistor R31, and an op amp A8 has its noninvertinginput coupled with the juncture between the impedance Z_(m) ' and theresistor R31. An inverting input to the op amp A8 is tied to its output,and the output from the op amp is applied to the inverting input to theop amp A6 through a resistor R34.

In operation of the circuit, signals on the receive channel at thenoninverting input 16 to the current source cause a current flow I_(zm)on the tip and ring leads, so that the local subscriber hears the farend subscriber's voice. Because of impedances on the tip and ring leads,which include the impedance Z_(m), the current flow on the leadsgenerates a voltage across the leads, which in turn causes an outputvoltage from the voltage detecting circuit 30. The output from thevoltage detecting circuit is applied to the inverting input to the opamp A6, and without more would be returned over the transmit channel tothe far end speaker as echo. An object of the hybrid circuit 40 is,therefore, to null any output from the op amp A6 which is in response tosignals from the signal source V1. However, upon the occurrence of nearend speech as represented by a voltage differential across the tip andring leads from the signal source V_(m), which also causes an outputfrom the voltage detecting circuit, it is desirable that the op amp A6generate an output which is proportional to the signal from the sourceV_(m) in order that the near end subscriber's voice signals may betransmitted to the far end subscriber.

In accomplishing the foregoing, the op amp A7 and its associatedresistors R30-R33 comprise an inverting current source for generating anoutput current I₀ ' in response to signals from the source V1. Therelationship between the resistors R30-R33 is such that the resistorsR30 and R33 are equal in value, the resistors R31 and R32 are equal, andthe resistors R30 and R33 are each significantly greater in value thanthe resistors R31 and R32. Under this condition, the output current I₀ 'through the resistor R31 is

    I.sub.0 '=-V1/R30.

As discussed with respect to the basic current source of FIG. 1, thecurrent I_(zm) applied to the load Z_(m) is

    I.sub.zm =V1R6/R1R2,

and in order to obtain a null or zero volt output from the op amp A6 inresponse to signals from the source V1 it is necessary that

    I.sub.zm Z.sub.m +I.sub.0 'Z.sub.m '=0.

To this end, the op amp A8 is merely a buffer amplifier and the op ampA6 an inverting summing amplifier. Thus, if in response to signals fromthe souce V1 the voltage applied across the resistor R34 by the op ampA8 equals the voltage generated across the resistor R28 by the voltagedetecting circuit, then the output from the op amp A6 will be nulled tozero.

In order that the lead current I_(zm) might be equal to the current-I_(O) ', the values of the resistors are adjusted such that R6/R1R2 isequal to 1/R30. Then, if the voltage detecting circuit is set for a 1:1conversion, the impedance Z_(m) ' equals the load impedance Z_(m) andR34 equals R28 for nulling to zero the output from the op amp A6 inresponse to signals on the receive channel. Consequently, speech signalson the receive channel are not returned to the far end subscriber asecho.

Note, however, that the op amp A6 does not interfere with thetransmission of speech signals from the near end to the far endsubscriber. To this end, upon the occurrence of an output from thesignal source V_(m), representative of the near end subscriber speaking,the voltage across the tip and ring leads is converted by the voltagedetecting circuit 30 and the op amp A6 to a voltage V_(A6) at the outputfrom the op amp. In this connection, the voltage V_(A6) is equal toV_(tr) R29/R28, where V_(tr) is the voltage generated across the tip andring leads by the source V_(m) and the voltage detecting circuit has again of 1. Obviously, the value of the resistor R29 may be set toprovide a desired gain for the op amp A6 in response to near end speechsignals.

Accordingly, the foregoing embodiment of the invention provides a uniqueelectronic hybrid circuit for interfacing between two and four-wire longdistance telephone transmission facilities. It is understood, of course,that the impedance Z_(m) ' may only be a best approximation of theexpected load impedance Z_(m), and that inaccuracies in theapproximation will result in some portion of the signal on the receivechannel being coupled with the transmit channel as echo. However, suchcoupling commonly occurs with two-coil hybrid circuits of theconventional type, and this characteristic of the electronic hybridcircuit is certainly no worse than that expected in conventionalcircuits. Advantageously, however, the electronic hybrid circuit is aneconomical, accurate and convenient replacement for conventional hybridcircuits, it affords a better match with impedances expected on the tipand ring leads so that echo, if any, is minimized and, because of thecurrent source 10, offers common mode rejection to longitudinal noise onthe tip and ring leads.

In FIG. 5 the various novel embodiments of the invention illustrated anddescribed in connection with FIGS. 1-4 have been combined to form aspecific product application. In particular, FIG. 5 illustrates what iscustomarily referred to as a 24V4 terminating set, which providestoll-grade interfacing between two and four-wire voice frequencytransmission facilities. The terminating set presents a fixed inputimpedance to the two-wire side of the facility, and either the same or adifferent fixed impedance at each of the transmit and receive inputports on the four-wire side of the facility.

The terminating set has receive and transmit channel input ports forinterfacing with the receive and transmit channels of a long distancetransmission facility, and connects with the tip and ring leads T and Rof a two-wire local transmission facility. In operation, a voice signalfrom a distant subscriber at the receive port is converted to a signalacross the tip and ring leads for being received by a local subscriber,and voice signals from the local subscriber across the tip and ringleads are coupled with the transmit port for being sent to the distantsubscriber, whereby two-way communication may occur. To this end, it isintended that an op amp A9 coupled with the receive port translate thesignal thereat into a voltage, and that a representation of the voltageappear across the load impedance Z_(m), which is the impedance of thelocal subscriber's hand set. It is also intended that any voltage V_(tr)across the tip and ring leads generated in response to a signal at thereceive port be blocked from returning to the transmit port, so thatecho is not heard by the distant subscriber. However, voltages generatesacross the tip and ring leads by the source V_(m), and which representvoice signals of the local subscriber, are to be coupled with thetransmit port, after being scaled by the op amp A6, for being sent tothe distant subscriber.

More particularly, all of the functions performed by the embodiments ofthe invention shown in FIGS. 1-4 have been combined into the terminatingset. The receive channel port of the set is defined at a transformer T1,the secondary winding of which has a resistor R35 connected thereacrossand to a noninverting input to the op amp A9. A pair of resistors R36and R37 control the gain of the op amp, so that the op amp imparts gainor loss to the signal from the receive channel. The op amp A9 takes theplace of the signal source V1 of FIGS. 1 and 4, and its output isapplied to the resistor R2 at the noninverting input to the currentsource 10. The outputs from the current source are coupled with the tipand ring leads and provide common mode rejection to longitudinal noisethereon.

To perform an impedance cancelling function, an impedance canceller 20of the type illustrated and described in FIG. 2 is connected at itsinputs across the tip and ring leads and to the junctures between theinductor L1 and the resistors R14 and R15, which represent an unwantedimpedance. The output from the impedance canceller is applied throughthe resistor 16 to the inverting input to the op amp A2 of the currentsource 10 so that, and as described with respect to FIG. 2, in responseto a metallic voltage on the tip and ring leads the current sourcegenerates on the leads the same current that would otherwise flowthrough the unwanted impedance in response to the metallic voltage,whereby the impedance appears as an open circuit to the metallicvoltage.

To generate a selected input impedance Z_(in) across the tip and ringleads, a voltage detecting circuit 30 has its inputs across the tip andring leads and its output coupled through the impedance Z27 with theinverting input to the op amp A1 of the current source 10. As describedwith respect to FIG. 3, connecting the impedance Z27 in this mannercauses the current source to generate a positive input impedance Z_(in),the value of which is determined by the value of the impedance Z27. Inthis connection, the current source then generates on the tip and ringleads, in response to a metallic voltage, an inverted output current ofthe same value that would flow through an impedance Z_(in), connectedacross the tip and ring leads, in response to the metallic voltage.

The electronic hybrid portion 40 of the terminating set, which enablesthe same to couple a four-wire voice frequency transmission facilitywith the tip and ring leads of a two-wire facility, comprises the opamps A7 and A8, their associated resistors R30-R34, and the impedancesZ_(m) ' and Z_(in) '. The inverting input to the op amp A7 is coupledthrough the resistor R30 to the juncture between the op amp A9 and theresistor R2, and the output from op amp A8 is applied through theresistor R34 to the inverting input to the op amp A6. In its operation,and as explained with respect to FIG. 4, the electronic hybrid controlsthe signal applied to the inverting input to the op amp A6 so thatvoltage signals across the tip and ring leads generated in response tosignals on the receive channel do not cause an output from the op amp,but so that signals across the leads in response to the source V_(m) do.The output from the op amp A6 is coupled through a resistor R38 to aprimary winding of a transformer T2, the secondary winding of whichforms the transmit port of the terminating set.

It should be noted that the electronic hybrid circuit 40 as shown inFIG. 5 differs from that in FIG. 4 in the inclusion of the impedanceZ_(in) '. However, in view of the impedance Z_(in) being generated bythe current source in parallel with the load impedance Z_(m), theimpedance Z'_(in) must be included in parallel with the impedance Z'_(m)to properly balance the electronic hybrid circuit to preventintroduction of echo on the transmit channel.

While embodiments of the invention have been described in detail,various modifications and other embodiments thereof may be devised byone skilled in the art without departing from the spirit and the scopeof the invention, as defined in the appended claims.

What is claimed is:
 1. A current source for interfacing between a sourceof voltage signals which vary in value and a two-wire transmissionsystem on which longitudinal voltages are at least occasionally present,such as the tip and ring leads of a telephone pair, for generating acontrolled current flow through the wires in accordance with the valueand polarity of voltage signals from the source of voltage signals andfor providing common mode rejection to longitudinal voltages on thewires, said current source having first and second outputs each forconnection with a separate one of the wires and comprising first andsecond voltage amplifier means each having an input, an output and anoutput impedance connected between the amplifier means output and aseparate one of said current source outputs, said first voltageamplifier means output being connected through its output impedance withsaid current source first output; means for connecting said firstamplifier means output with said second amplifier means input so thatvoltage signals at said first amplifier means input control the voltagesgenerated at each of said amplifier means outputs; feedback meanscoupled between said current source first output and said firstamplifier means input; circuit means coupled between said current sourceoutputs and said first amplifier means input, said feedback means andsaid circuit means coupling signals at said current source outputs fromthe wires of the transmission system to said first amplifier meansinput, said feedback means and said circuit means in response to equallongitudinal voltages on the wires coupling to said first voltageamplifier means input signals which cancel to zero at said first voltageamplifier means input so that said first and second voltage amplifiermeans do not generate voltages at their outputs in response to equallongitudinal voltages on the wires, whereby said current source hascommon mode rejection to equal longitudinal voltages on the wires; andmeans for coupling the source of voltage signals with said first voltageamplifier means input to generate at said current source outputs andthrough the wires current flows having values and directions inaccordance with the value and polarity of voltage signals from thesource of voltage signals.
 2. A current source as in claim 1, whereinsaid first and second voltage amplifier means each generate zero voltsor reference potential at their outputs in response to equallongitudinal voltages on the wires.
 3. A current source as in claim 2,wherein said first and second voltage amplifier means generatesubstantially equal and opposite value voltages at their outputs inresponse to noncancelling signals at said first amplifier means input.4. A current source as in claim 2, wherein said first and second voltageamplifier means generate substantially equal value and oppositedirection current flows through their output impedances, and thereforeat said current source outputs, in response to and having a value inaccordance with noncancelling signals at said first amplifier meansinput.
 5. A current source as in claim 2, wherein said circuit meanscomprises first and second impedance means coupled in series betweensaid current source outputs and at a juncture therebetween with saidfirst amplifier means input.
 6. A current source as in claim 5, furtherincluding third impedance means coupled between said current sourcesecond output and reference potential, said feedback means includingfourth impedance means coupled between said current source first outputand said first amplifier input, and wherein said first and secondamplifier means output impedances have equal values, said first andsecond impedances have equal values, and said third and fourthimpedances have equal values, so that said current source has a balancedlongitudinal impedance between its outputs.
 7. A current source as inclaim 1, wherein said feedback means and said circuit means couplecurrent signals to said first voltage amplifier means input, in responseto equal longitudinal voltages on the wires, which are equal andopposite in value and cancel.
 8. A current source having first andsecond outputs each for connection with a separate one of the wires of atwo-wire transmission system on which longitudinal voltages are at leastoccasionally present, such as with the tip and ring leads of a telephonepair, for generating a controlled current flow through the wires inaccordance with the value and polarity of voltage signals from a sourceof voltage signals which vary in value and for providing common moderejection to longitudinal voltages on the system, said current sourcecomprising first and second current generators each having an input andan output, said first current generator output being coupled both withsaid current source first output and with said second current generatorinput and said second current generator output being coupled with saidcurrent source second output; means for coupling the source of voltagesignals with said first current generator input, said current generatorsin response to voltage signals from the voltage signal source at saidfirst current generator input providing at said current source outputsand therefore on the wires equal and opposite direction current flowshaving values and directions in accordance with the value and polarityof the voltage signals from the voltage signal source; and feedbackmeans coupled between said current source outputs and said first currentgenerator input for coupling signals at said outputs from the wires withsaid first current generator input, said feedback means in response toequal longitudinal voltages on the wires coupling to said first currentgenerator input signals which cancel to zero at said first currentgenerator input so that said first and second current generators do notgenerate current flows at their outputs in response to equallongitudinal voltages on the wires, whereby said current source exhibitscommon mode rejection to longitudinal voltages on the wires.
 9. Acurrent source as in claim 8, said first and second current generatorscomprising first and second voltage amplifiers each having an input, anoutput and an output impedance connected between the voltage amplifieroutput and the respective current source output, said feedback meansincluding first circuit means coupled between said current sourceoutputs and with said first voltage amplifier input and second circuitmeans coupled between said current source first output and said firstvoltage amplifier input, said first and second circuit means generatingequal and opposite value signals at said first voltage amplifier inputin response to equal longitudinal voltages on the wires and at saidcurrent source outputs, said equal and opposite signals cancelling atsaid first voltage amplifier input so that said first and second voltageamplifiers generate zero volts or ground potential at their outputs inresponse to equal longitudinal voltages on the wires.
 10. A currentsource for interfacing between a source of voltage signals which vary invalue and a two-wire transmission system of a type which exhibitslongitudinal voltages and a metallic impedance and voltage across thewires, said current source having first and second outputs each forconnection with a separate one of the wires for generating through thewires and the metallic impedance a controlled current flow having avalue and direction in accordance with the value and polarity of voltagesignals from the source of voltage signals and for exhibiting commonmode rejection to longitudinal voltages on the wires, said currentsource comprising a first operational amplifier (op amp) A1 connected ata noninverting input thereto with reference or ground potential; aresistor R1 connected at one end to an output from said op amp A1 andcoupled at an opposite end with sai current source first output; asecond op amp A2 connected at a noninverting input thereto withreference potential; a resistor R3 connected between the inverting inputto and an output from said op amp A2; a resistor R4 connected betweenthe output from said op amp A2 and an inverting input to said op amp A1;a resistor R5 connected between the inverting input to and the outputfrom said op amp A1; a resistor R6 connected between said opposite endof said resistor R1 and the inverting input to said op amp A2; an op ampA3 connected at a noninverting input thereto with reference or groundpotential; a resistor R8 connected between the output from said op ampA1 and an inverting input to said op amp A3; a resistor R9 connectedbetween the inverting input to and an output from said op amp A3; aresistor R10 connected at one end with the output from said op amp A3and coupled at an opposite end with said current source second output; aresistor R11; a resistor R12 connected in series with said resistor R11between said opposite ends of said resistors R1 and R10 and at ajuncture therebetween with the inverting input to said op amp A1; aresistor R13 connected betwenen said opposite end of said resistor R10and ground; and means for coupling the source of voltage signals withthe inverting input to said op amp A1.
 11. A current source as in claim10, further including a coupling capacitor C1 connected between saidopposite end of said resistor R1 and said current source first output,and a coupling capacitor C2 connected between said opposite end of saidresistor R10 and said current source second output.
 12. A current sourceas in claim 10, wherein the values of said resistors R1, and R3 throughR6, R8 through R13 are such that:

    R1=R10,

    R3=R4=R5=R6=R13,

    R8=R9, and

    R11=R12=2R6=2R13,

whereby equal and balanced longitudinal impedances result between saidcurrent source outputs and ground and said op amps A1 and A3 generateequal voltages at their outputs in response to equal longitudinalvoltages on the wires and at said current source outputs, so that saidcurrent source provides common mode rejection to longitudinal voltages.13. A current source as in claim 12, wherein the currents I through saidresistors R4, R11 and R12 at the inverting input to said op amp A1 sumto zero in response to equal longitudinal voltages at said currentsource outputs, such that

    I.sub.R11 +I.sub.R12 +I.sub.R4 =0,

whereby the output voltage from each of said op amps A1 and A3 is zerovolts in response to equal longitudinal voltages and said current sourcedecreases the longitudinal voltages on the wires and generates abalanced impedance path to ground from each of its outputs, and whereinthe longitudinal impedance Z_(CS1) from said current source first outputto ground is: ##EQU3## the longitudinal impedance Z_(CS2) from saidcurrent source second output to ground is: ##EQU4## whereby the totallongitudinal impedance Z_(t) to ground presented at said current sourceoutputs is:

    Z.sub.t =Z.sub.CS1 Z.sub.CS2 /(Z.sub.CS1 +Z.sub.CS2),

and wherein the metallic input impedance Z_(in) across said currentsource outputs is equal to:

    Z.sub.in =1/(1/R1+1/R6-R5/R1R6),

whereby said currrent source generates a selected input impedance to thetwo-wire system.
 14. A current source as in claim 13, wherein the valuesof said resistors R1, R5 and R6 are selected so that:

    R1+R6=R5,

whereby said current source generates an infinite input impedance Z_(in)at its outputs to the two-wire system.
 15. A current source as in claim13, further including a resistor R2 connnected at one end thereof withthe inverting input to said op amp A2 and wherein said means forcoupling the source of voltage signals with the inverting input to saidop amp A1 comprises means for coupling the source of voltage signalswith an opposite end of said resistor R2, the source of voltage signalshaving an output voltage V1, so that upon connection of said resistor R2with the source of voltage signals the output from said op amp A1 causesgeneration of a noninverted output current I₀ thereat, and the outputfrom said op amp A3 causes generation of an inverted output current -I₀thereat, said output currents having an absolute value equal to:

    I.sub.0 =V1R5/R1R2.


16. A current source as in claim 13, further including a resistor R7connected at one end thereof with the inverting input to said op amp A1and wherein said means for coupling the source of voltage signals withthe inverting input to said op amp A1 comprises means for coupling thesource of voltage signals with an opposite end of said resistor R7, thesource of voltage signals having an output voltage V2, so that uponconnection of said resistor R7 with the source of voltage signals theoutput from said op amp A1 causes generation of an inverted outputcurrent -I₀ thereat, and the output from said op amp A3 causesgeneration of a noninverted output current I₀ thereat, said outputcurrents having an absolute value equal to:

    I.sub.0 =V2R5/R1R7.


17. A current source as in claim 13, wherein the source of voltagesignals includes a first voltage source having an output V1 which variesin value and a second voltage source having an output V2 which varies invalue, and wherein said means for coupling the source of voltage signalswith the inventing input to said op amp A1 comprises a resistor R2connected at one end with the inverting input to said op amp A2 andconnectable at an opposite end with the first voltage source having theoutput voltage V1, and a resistor R7 connected at one end with theinverting input to said op amp A1 and connectable at an opposite endwith the second voltage source having the output voltage V2, so thatupon connection of said resistors R2 and R7 with their respectivevoltage sources the outputs from said op amps A1 and A3 cause generationof equal and opposite direction output currents I₀ thereat, each saidoutput current having an absolute value equal to:

    I.sub.0 =V1R5/R1R2-V2R5/R1R7.


18. A current source as in claim 17, wherein a current flow I_(rm)provided at the outputs from said current source across the wires isequal to:

    I.sub.rm =V1R6/R1R2-V2R6/R1R7,

wherein a portion I_(leak) of the current I₀ generated at the outputfrom the op amp A1 which flows through said resistor R6 is euqal to:

    I.sub.leak =1/R1+1/R6-R5/R1R6,

and wherein the values of said resistors R1, R5 and R6 are such that R5equals the sum of R1 plus R6, so that I_(leak) is substantially equal tozero and I₀ is substantially equal to I_(rm).
 19. A current source as inclaim 10, wherein the values of said resistors R1, R6 and R10 throughR13 are such that:

    R1=R10,

    R6=R13, and

    R11=R12=2R6=2R13,

whereby equal and balanced longitudinal impedances result between saidcurrent source outputs and ground.
 20. A current source as in claim 10,wherein the values of said resistors R1, R3 through R6, R8 through R13are selected such that the currents I through said resistors R4, R11 andR12 at the inverting input to said op amp A1 sum to zero in response toequal longitudinal voltages at said current source outputs, such that

    I.sub.R11 +I.sub.R12 +I.sub.R4 =0

whereby the output voltage from each of said op amps A1 and A3 is zerovolts in response to equal longitudinal voltages and said current sourcedecreases the longitudinal voltages.
 21. A current source as in claim10, wherein the values of said resistors R1, R3 through R6 and R8through R13 are selected such that said current source generates abalanced impedance path to ground from each of its outputs and so thatthe longitudinal impedance Z_(CS1) from said current source first outputto ground is: ##EQU5## the longitudinal impedance Z_(CS2) from saidcurrent source second output to ground is: ##EQU6## whereby the totallongitudinal impedance Z_(t) to ground presented at said current sourceoutputs is:

    Z.sub.t =Z.sub.CS1 Z.sub.CS2 /(Z.sub.CS1 +Z.sub.CS2).


22. A current source as in claim 10, wherein the values of saidresistors R1, R3 through R6 and R8 through R13 are chosen so that themetallic input impedance Z_(in) across said current source outputs isequal to:

    Z.sub.in =1/(1/R1+1/R6-R5/R1R6),

whereby said current source generates a selected input impedance to thetwo-wire system.
 23. A current source as in claim 21, further includinga resistor R2 connected at one end thereof with the inverting input tosaid op amp A2 and wherein said means for coupling the source of voltagesignals with the inverting input to said op amp A1 comprises means forcoupling the source of voltage signals with an opposite end of saidresistor R2, the source of voltage signals having an output voltage V1,and wherein the values of said resistors R1 through R6 and R8-R13 areselected so that upon connection of said resistor R2 with the source ofvoltage signals the output from said op amp A1 causes generation of anoninverted output current I₀ thereat, and the output from said op ampA3 causes generation of an inverted output current -I₀ thereat, saidoutput current having absolute values equal to:

    I.sub.0 =V1R5/R1R2.


24. A current source as in claim 10, further including a resistor R7connected at one end thereof with the inverting input to said op amp A1and wherein said means for coupling the source of voltage signals withthe inverting input to said op amp A1 comprises means for coupling thesource of voltage signals with an opposite end of said resistor R7, thesource of voltage signals having an output voltage V2, and wherein thevalues of said resistors R1 and R3 through R13 are selected so that uponconnection of the resistor with the source of voltage signals the outputfrom said op amp A1 causes generation of an inverted output current -I₀thereat, and the output from said op amp A3 causes generation of anoninverted output current I₀ thereat, said output currents havingabsolute values equal to:

    I.sub.0 =V2R5/R1R7.


25. A current source as in claim 10, wherein the source of voltagesignals includes a first voltage source having an output V1 which variesin value and a second voltage source having an output V2 which varies invalue, and wherein said means for coupling the source of voltage signalswith the inverting input to said op amp A1 comprises a resistor R2connected at one end with the inverting input to said op amp A2 andconnectable at an opposite end with the first voltage source having theoutput voltage V1, and a resistor R7 connected at one end with theinverting input to said op amp A1 and connectable at an opposite endwith the second voltage source having the output voltage V2, and whereinthe values of said resistors R1 through R13 are selected so that uponconnection of said resistors R2 and R7 with their respective voltagesources the outputs from said op amps A1 and A3 cause generation ofequal and opposite direction output currents I₀ thereat, each saidoutput current having an absolute value equal to:

    I.sub.0 -V1R5/R1R2-V2R5/R1R7.


26. A current source as in claim 25, wherein the values of saidresistors R1 through R13 are further selected so that the current flowI_(rm) provided at the outputs from said current source across the wiresis equal to:

    I.sub.rm =V1R6/R1R2-V2R6/R1R7,

wherein a portion I_(leak) of the current I₀ generated at the outputfrom the op amp A1 which flows through said resistors R6 is equal to:

    I.sub.leak =1/R1+1/R6-R5/R1R6,

and wherein the values of said resistors R1, R5 and R6 are such that R5equals the sum of R1 plus R6, so that I_(leak) is substantially equal tozero and I₀ is substantially equal to I_(rm).